fix(07): CR-02 correct cron expression for pollIntervalMin >= 60

Values >=60 in the cron minute field silently misbehave (*/60 fires once per
hour, */90 fires once per hour, etc.). Use the hours field for intervals >=60:
  <60 min  → */N * * * *
  >=60 min → 0 */H * * * (H = floor(N/60))
Also adds @Max(1440) to DkvConfigDto to bound the field at 24 h.

apps/api/src/dkv/dto/dkv-config.dto.ts

@@ -72,11 +72,13 @@ export class DkvConfigDto {
 
   /**
    * Polling interval in minutes. Minimum 5 minutes to prevent DoS via excessive polling
-   * (T-07-05, UI-SPEC minimum = 5).
+   * (T-07-05, UI-SPEC minimum = 5). Maximum 1440 (24 h) — values above 59 use the cron
+   * hours field (must be a factor of 24 for predictable hourly+ behaviour).
    */
   @IsOptional()
   @IsInt()
   @Min(5)
+  @Max(1440)
   pollIntervalMin?: number;
 
   /**